There has been great activity in the field of removing NO.sub.x from combustion zone gases. Much of the work has been done on the removal of SO.sub.x and NO.sub.x from a gas stream derived from coal and residual oil burning furnaces of electric power generating stations. There are many examples of this process stream being purified of the SO.sub.x and NO.sub.x, but that is not a part of the present invention. When both SO.sub.x and NO.sub.x are present, many of the schemes handle the SO.sub.x in one reactor and NO.sub.x in a second reactor, after SO.sub.x has been removed. This process and its many variations are not particularly pertinent to the present invention.
U.S. Pat. Nos. 4,182,745 and 4,282,115 are of interest to the present invention. U.S. Pat. No. 4,182,745 issued to Nishida, et al. describes a typical method used for removal of nitrogen oxide by selective conversion by reaction of the nitrogen oxide with ammonia in the presence of oxygen. This process is described and other background information given in column 1, lines 10 through 51.
The uniqueness of the Nishida et al. catalysts is stated also in column 1, lines 53 through 65. The catalysts which are useful in this process are the heteropolyacids and their salts are also identified as being applicable, those are enumerated in column 2, lines 28 through 54.
There are many points of difference between the Nishida et al. process reference (known broadly as the SCR process) and the process of the present invention. First is that the present invention uses no ammonia, whereas, the SCR process uses ammonia as a selective reducing agent. The second point of difference is that the catalyst and adsorbent of the present invention operate at less than 300.degree. C., which is a typical commercially economic condition. The catalyst in question in the SCR process, must operate above 350.degree., and the single example shows it operating at 400.degree. C., thus entailing a substantial commercial liability for heating the flue gas or heat exchanging after the reduction. In this process the permissible space velocity is 3,000 to 8,000 whereas in the present invention the space velocity is 12,000 to 18,000 making for lower capital costs.
U.S. Pat. No. 4,282,115 issued to Atsukawa, et al. as described in the abstract, uses ammonia as a reducing agent for the reduction of the nitrogen oxides. The novel feature of this patent is that a unique support, calcium silicate, is used and is purported to provide improved resistance to sulfur poisoning. Thus, the thrust of this patent is one of an improved support. Column 3, lines 47 through 67 and column 4, lines 1 through line 66 list prior art.
These SCR cases describe the prior art as it pertains to the use of ammonia as a selective reducing agent for the nitrogen oxide in the presence of oxygen. Other reducing gases such as hydrogen, methane and carbon monoxide are mentioned as not being as selective as ammonia. One of the major problems, however, with the use of ammonia, is the high temperature that is required and the fact that the nitrogen oxide is removed only to the extent of 75 to 95% and not the 100% removal accomplished in the present invention. Furthermore, the ammonia may not be completely reacted with the result that it would, itself be discharged to the atmosphere where it would produce harmful pollution.
A further prior art is a paper which was presented by Shell research of Amsterdam (the Netherlands) as a part of the proceedings of the 1989 joint EPA-EPRI Symposium on stationary combustion NO.sub.x control. This paper discloses that the catalyst is sensitive to sulfur and, as shown on page 2 of the paper, the NO.sub.x conversion is only 60% to 80%. It also is of note that the catalyst is very susceptible to moisture content with the result that moisture tends to deactivate the catalyst. All flue or exhaust gases would contain 10 or more percent of moisture from the inlet air as well as the combustion of the fuel.
The foregoing prior art all are processes which are very closely related to the general process SCR which is the abatement of NO.sub.x using ammonia as the reducing gas. Various prior art show the problems with the process and through it is very different from the present process, are referred to because of the fact that it does remove nitrogen oxide but by a process which is vastly inferior and is substantially different from the process of this invention.
A further prior art of this same process is given in Industrial and Engineering Chemical Research. Issue 29 in the 1990 volume, pg. 1985-1989: This process as described in the introduction on page 1985-1989: This process as described in the introduction on page 1985 is very similar to the two patented processes previously described, except that amorphous chromia is used as catalyst instead of the lanthanum and titanium oxides of the previous references. Furthermore, in this test, there is some very serious doubt thrown on the validity and commercial utility of the data because the gases that are used in the denitrogenation are all anhydrous, whereas any commercial process except in very rare cases, would have water vapor in it.
Other types of nitrogen oxide abatement process will be referred to herein. The first is one entitled "Enhancement Effect of Magnesium Plus Two Ions Under Direct Nitrate Oxide Decomposition Over Supported Palladium Catalyst". This is presented in Applied Calalysis 65, 1990, Letters, pg. 11-Letters page 15. The process is briefly described and superiority is claimed in the introduction on page L11. In describing prior work, certain precious metals catalyst were described but then it was shown that they were not active until temperatures exceeded 500.degree. C. and, preferably, were in the range of 700.degree.-800.degree. C. The superiority of the catalyst presented and described in this reference, which is a magnesium promoted material, is indicated by the fact that it will operate at a temperature in excess of 650.degree. C. The process does not use ammonia, but the conversion of nitrogen oxide and abatement of nitrogen oxide at 550.degree. C. does not exceed 23% and at 650.degree. C. does not exceed 50%. These data are shown in table 1 on page L-13. It is clear that this process is both expensive from the standpoint of temperature requirements and reheat fuel, furthermore is very poor from the standpoint of nitrogen oxide abatement.
A further process, described as the NOXOL process, was briefly described in the "Chemical and Engineering News" in their science technology concentrates, Oct. 21, 1991, pg. 20. In this process, activated alumina granules impregnated with sodium carbonate were used to adsorb both nitrogen oxide and sulfur dioxide. The nitrogen oxide was further processed by desorbing from the adsorbent, and recycling to the furnace to which was added a small amount of methane (natural gas) under which conditions the amount of nitrogen oxide abatement increases from approximately 6% to approximately 90%. This process is under investigation at a commercial installation of the Ohio Power Company at a location which was not identified. The efficacy of this process is not given since the degree to which the nitrogen oxide is removed from the gases by the sodium carbonate alumina adsorbent is not given. The degree to which these nitrogen oxides are regenerated from the sodium carbonate is also not given, but it would be expected that for good removal, very high temperatures would be involved, that is above 600.degree. C. It is not stated, but it would be expected that if the sulfur dioxide is adsorbed by the sodium carbonate, sodium sulfite would be formed which would, in the presence of the oxygen in the gas stream, be converted to sulfate and regeneration would be essentially impossible except at extremely high temperatures, probably above 1,000.degree. C. No report has recently been received of the performance at this commercial site, probably because it is too early to get any indication of performance.
A still further procedure for NO.sub.r abatement is given in "Industrial Engineering Chemistry Product Research and Development" of 1983, 21, pg. 405-408. This process also has serious shortcomings one of which is that the test was made with no oxygen in the gas stream, which, of course, immediately brings into question its capability of removing nitrogen oxides in an atmosphere containing oxygen. Furthermore, temperatures of operation and testing were in the range of 600.degree.-700.degree. C. The information just quoted is given in the introduction to the paper on page 405, whereas the temperature of operation is given in the second column on page 406. Also, at the bottom of this column, the statement is made to have a high conversion when oxygen is present, the temperature must be raised to 750.degree. C. From the standpoint of a practical commercial operation, this is economically unsound.
A still further reference is to a paper in "Industrial Engineering Chemistry Product Research and Development", 1983, line 21 pg 56-59. This process is described in the introduction and comprises a catalyst either nickel oxide or cobalt oxide, supported on activated carbon. The activated carbon was used for the reduction. A description of the process is given briefly in the abstract on page 56 and in the introduction on pages 56 and 57. This process is one in which a catalyst is consumed in the course of the removal of the nitrogen oxide. The NO.sub.x reacts with the carbon forming carbon-dioxide, and, simultaneously, the catalyst is being destroyed. It is obviously a very poor solution to the problem and its commercial development has obviously not been achieved since it has been ten years since it was originally proposed in the periodical.
A further reference is given in "Energy and Fuels", 2989, Vol. III, pg 740-743. The title of the paper is "Control of NO.sub.x Emissions by Selective Catalytic Reductions With Hydrogen Over Hydrophobic Catalysts", by L. Fu and K. T. Schuang. The process is described both in the abstract and in the introduction, with the basic concept being that a hydrophobic support, which in this case is di vinyl-benzene-styrene resin, and the catalytic metals, are platinum, platinum plus ruthenium, palladium, ruthenium alone, and gold. The conversion in this process was reported to be 60-80%, but, in the presence of oxygen, this was sharply reduced.